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United States Patent |
5,350,637
|
Ketcham
,   et al.
|
September 27, 1994
|
Microlaminated composites and method
Abstract
Microlaminated composite articles are made by combining one or more sheets
of flexible sintered crystalline ceramic foil with one or multiple
inorganic substrate layers, e.g., of metal foil, to form a stack which is
then heated below the melting temperatures of the foil and substrate
layers, and under slight or moderate pressure, to provide a well-bonded
composite article which is essentially free of interlaminar cementing or
sealing materials.
Inventors:
|
Ketcham; Thomas D. (Big Flats, NY);
Share; Leroy S. (Corning, NY);
St. Julien; Dell J. (Watkins Glen, NY)
|
Assignee:
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Corning Incorporated (Corning, NY)
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Appl. No.:
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968667 |
Filed:
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October 30, 1992 |
Current U.S. Class: |
428/539.5; 156/89.28; 264/DIG.36; 264/DIG.57; 428/433; 428/545 |
Intern'l Class: |
B32B 018/00 |
Field of Search: |
264/58,DIG. 36,DIG. 57,60
428/545,539.5,432,688,428,433,689
419/5,8
|
References Cited
U.S. Patent Documents
2405529 | Aug., 1946 | Smith | 175/41.
|
3195030 | Jul., 1965 | Herzog et al. | 317/258.
|
3490887 | Jan., 1970 | Herzog et al. | 65/33.
|
4340436 | Jul., 1982 | Dubetsky et al. | 156/89.
|
4656071 | Apr., 1987 | Virkar | 428/36.
|
4677009 | Jun., 1987 | Virkar | 428/36.
|
4714257 | Dec., 1987 | Heinrich et al. | 277/1.
|
4725333 | Feb., 1988 | Leedecke et al. | 156/630.
|
4806295 | Feb., 1989 | Trickett et al. | 264/59.
|
4835656 | May., 1989 | Kitahara et al. | 361/321.
|
4849292 | Jul., 1989 | Mizunoga et al. | 428/433.
|
4868711 | Sep., 1989 | Hirama et al. | 361/321.
|
5059582 | Oct., 1991 | Chung | 505/1.
|
5089455 | Feb., 1992 | Ketcham et al. | 501/104.
|
5098494 | Mar., 1992 | Reisman | 156/89.
|
Foreign Patent Documents |
0123186 | Oct., 1984 | EP | .
|
0441528A1 | Aug., 1991 | EP | .
|
63-278833 | Nov., 1988 | JP | .
|
Other References
J. G. Pepin et al., "Electrode-Based Causes of Delaminations in Multilayer
Ceramic Capacitors", J. Am. Ceram. Soc., 72 [12] 2287-91 (1989).
K. G. Shaw, "Fabrication of Plasma Sprayed Composite Structures", Proc.
American Society for Composites, Sixth Technical Conference, pp. 145-153,
Technomic Publishing Company, Lancaster, Penn. (1991).
H. C. Cao et al., "On Crack Extension in Ductile/Brittle Laminates", Acta.
metall. mater. vol. 39, No. 12, 2997-3005, (Dec. 1991).
J. H. Givens et al., "Metal Matrix Composites: Titanium Nitride Films as
Microlaminate Reinforcements", Metal & Ceramic Matrix Composites:
Processing, Modeling & Mechanical Behavior, (R. B. Bhagat et al. Ed.), pp.
587-596, The Minerals, Metals & Materials Soc., 1990.
W. J. Clegg et al., "A simple way to make tough ceramics", Nature vol. 347,
pp. 455-457 (4 Oct. 1990).
D. B. Marshall et al., "Enhanced Fracture Toughness in Layered
Microcomposites of Ce-ZrO2 and A12O3", J. Am. Ceram. Soc., 74 [12] 2979-87
(1991).
M. Yasrebi et al., "Biomimetic Processing of Ceramics and Ceramic-Metal
Composites" Mat. Res. Soc. Symp. Proc. vol. 180, pp. 625-635 (1990).
A. Bose et al., "MIMLCs: Microinfiltrated macrolaminated composites . . .",
Advanced Materials and Processes, Jul. 1991, pp. 19-22.
M. C. Shaw et al., "Bridging Processes in Metal-Reinforced Ceramics", Mat.
Res. Soc. Symp. Proc. vol. 170, pp. 25-31, Materials Research Society
(1990).
A. N. Virkar et al., "Strengthening of Oxide Ceramics by
Transformation-Induced Stresses", J. Am. Ceram. Soc., 70 [3] 164-170
(1987).
R. A. Cutler et al., "Strength Improvement in Transformation-Toughened
Alumina by Selective Phase Transformation", J. Am. Ceram. Soc. 70 [10]
714-718 (1987).
|
Primary Examiner: Walsh; Donald P.
Assistant Examiner: Chi; Anthony R.
Attorney, Agent or Firm: van der Sterre; Kees
Claims
We claim:
1. A method for making a microlaminated composite article which essentially
consists the steps of:
providing at least one sheet of flexible sintered crystalline ceramic foil;
providing at least one inorganic substrate layer of a metal, intermetallic
compound or crystalline ceramic material;
stacking the ceramic foil on the substrate layer; and
heating the stack under pressure at a temperature and for a time at least
sufficient to obtain permanent bonding of the ceramic foil sheet directly
to the inorganic substrate layer.
2. A method for making a microlaminated composite article which essentially
consists the steps of:
providing a plurality of sheets of flexible sintered crystalline ceramic
foil;
providing one or a plurality of inorganic substrate layers of a metal,
intermetallic compound or crystalline ceramic material;
forming a sheet stack wherein at least some of the sheets of ceramic foil
are in contact with one or more of the inorganic substrate layers; and
heating the sheet stack under pressure to a temperature and for a time at
least sufficient to obtain permanent bonding of the ceramic foil sheets
directly to the one or more inorganic substrate layers in contact
therewith.
3. A method in accordance with claim 2 wherein the substrate layers are
sheets of flexible metal, intermetallic or ceramic foil, and wherein the
sheet stack is characterized by at least some alternate interleaving of
the first sheets and the second sheets or layers.
4. A method in accordance with claim 1 wherein the pressure does not exceed
about 500 kPa.
5. A method for making a composite article which essentially consists the
steps of:
providing one or more sheets of a flexible sintered crystalline ceramic
foil;
providing one or more inorganic substrate sheets of metallic or
intermetallic composition;
forming the sheets of foil and substrate sheets into a sheet stack wherein
one or more sheets of the ceramic foil are positioned adjacent one or more
of the inorganic substrate sheets;
heating the ceramic foil and substrate sheets to a temperature in the range
from about 300 .degree. C. below the lowest sintering temperature of the
ceramic foil and inorganic substrate sheets to just below lowest melting
temperature of the ceramic foil and inorganic substrate sheets; while
applying pressure to the sheet stack, said pressure being at least
sufficient to insure full surface contact between the foil and substrate
sheets;
said heating being continued for a time at least sufficient to obtain
direct physical bonding of the ceramic foil to the inorganic substrate
sheets.
6. A method in accordance with claim 5 wherein a plurality of sheets of
ceramic foil and a plurality of inorganic substrate sheets are provided,
wherein the inorganic substrate sheets include sheets of metallic or
intermetallic foil, and wherein at least some of the ceramic foil sheets
are interleaved with the sheets of metallic or intermetallic foil.
7. A method in accordance with claim 6 wherein the ceramic foil has a
composition selected from the group consisting of zirconia, stabilized or
partially stabilized zirconia, hafnia, alumina, silica, titania, mullite,
spinel, chromium oxide, sialon, hasicon, silicon carbide, titanium
carbide, silicon nitride, titanium nitride, zircon, zirconium carbide,
titanium diboride, and combinations thereof.
8. A method in accordance with claim 7 wherein the ceramic foil has a
composition selected from the group consisting of stabilized zirconia and
partially stabilized zirconia, and wherein the zirconia includes at least
one stabilizer selected from the group consisting of alkaline earth
oxides, rare earth oxides, and combinations thereof.
9. A method in accordance with claim 7 wherein the ceramic foil has a
thickness in the range of about 10-100 micrometers.
10. A method in accordance with claim 9 wherein the ceramic foil has a
thickness in the range of about 25-50 micrometers.
11. A method in accordance with claim 5 wherein the inorganic substrate
sheets are metal foil sheets having a composition selected from the group
consisting of ferrous metals, copper, aluminum, nickel, nickel-based
alloys, titanium, molybdenum, niobium, tungsten, tantalum, chromium, and
chromium-based alloys.
12. A method in accordance with claim 11 wherein the pressure applied to
the foil sheets is in the range of 1-50 kPa.
13. A microlaminated composite article comprising:
a plurality of layers of ceramic foil partly or wholly interleaved by a
plurality of metallic or intermetallic substrate layers, wherein:
the layers of ceramic foil are directly bonded to the substrate layers in
contact therewith at ceramic foil-substrate interfaces therebetween;
the layers of ceramic foil and the substrate layers have thicknesses not
exceeding about 250 micrometers, and at least one dimension of length or
breadth in excess of 1 cm;
the ceramic foil layers and metallic substrate layers are essentially free
of pin-hole defects the ceramic foil-substrate interfaces are essentially
free of added adhesive interlayer material.
14. A composite article in accordance with claim 13 wherein the ceramic
foil has a composition selected from the group consisting of zirconia,
stabilized or partially stabilized zirconia, hafnia, alumina, silica,
titania, mullite, spinel, chromium oxide, sialon, nasicon, silicon
carbide, titanium carbide, silicon nitride, titanium nitride, zircon,
zirconium carbide, titanium diboride, and combinations thereof.
15. A composite article in accordance with claim 13 wherein the ceramic
foil has a composition selected from the group consisting of stabilized
zirconia and partially stabilized zirconia, and wherein the zirconia
includes at least one stabilizer selected from the group consisting of
alkaline earth oxides, rare earth oxides, and combinations thereof.
16. A composite article in accordance with claim 14 wherein the ceramic
foil has a thickness in the range of about 10-100 micrometers.
17. A composite article in accordance with claim 16 wherein the ceramic
foil has a thickness in the range of about 25-50 micrometers.
18. A composite article in accordance with claim 14 wherein the inorganic
substrate sheets are metal foil sheets having a composition selected from
the group consisting of ferrous metals, copper, aluminum, nickel,
nickel-based alloys, titanium, molybdenum, niobium, tungsten, tantalum,
chromium, and chromium-based alloys.
Description
BACKGROUND OF THE INVENTION
The present invention relates to composite articles formed of two or more
inorganic materials of differing composition and properties, and more
particularly to so-called "microlaminated" composites formed by the
lamination of sheets of such inorganic materials.
Composites are often made to create a beneficial mix of the properties of
dissimilar materials that can be unobtainable in a single homogeneous
material. Two well-known examples of a type of composite structure using a
layering of two materials to obtain such a mix of properties are "Samurai
Swords" and "Damascus Steel". In these composites, hard but brittle
carbide layers are interspersed with softer, ductile, tougher steel
layers. In the Samurai case, these layers are created by repeated
hammering and folding during the manufacture of the sword.
Newer varieties of metal matrix (ductile) composites reinforced with
metallic and non-metallic fibers, whiskers, platelets and particles have
also been developed. The reinforcing agents in these composites are
intended to impart greater stiffness, higher yield strength or ultimate
tensile strength, and/or higher creep resistance to the matrix material.
Various methods have been employed to fabricate such composites. In the
case of chopped fibers whiskers, platelets, or the like, mixtures of
components may be melt processed or sintered to incorporate the
reinforcing agents. Long fibers can be laid between metal sheets and the
metal sheets deformed around the fibers to obtain a densified composite,
although for large fibers a great deal of plastic deformation is necessary
to make the matrix material flow around the fiber to make a dense matrix.
Intermetallic compounds are also topics of composites research and,
although classed as semi-brittle materials, offer some advantages over
conventional alloys. However, while they can be reinforced with ceramic
materials, difficulties due to limited ductility and thermodynamic
incompatibility with ceramic reinforcing materials are more common,
particularly where substantial plastic deformation of the intermetallic is
required during manufacture.
Brittle materials such as glass and ceramics are even harder to process,
crystalline ceramics being particularly difficult to manufacture to net
shape at precise tolerances. Thus ceramics and glass have been largely
limited to low stress applications or to areas where their properties
(optical transparency, high dielectric constant, etc.) are essential.
Toughened fiber-reinforced ceramic matrix composites for high-stress
applications have recently been developed, but the high pressure/high
temperature consolidation processing needed for the manufacture of these
composites greatly adds to their cost, and the achievement of a truly
uniform distribution of reinforcing fibers therein remains difficult.
In broad aspect, composites can be classified into brittle/brittle,
brittle/semibrittle, and brittle/ductile categories, depending upon the
particular combinations of brittle materials (eg., ceramics) and ductile
materials (eg., metals) used in their construction. Layered or laminated
composites, familiar in products such as capacitors and multilayer
electronic substrates, comprise a well-recognized subgroup of composite
structures.
The physical processes involved in the manufacture of well-bonded laminated
composites include co-sintering, hot pressing, metal infiltration, and
diffusion bonding. Co-sintering is employed in the fabrication of
ceramic/ceramic and metal/ceramic composites, with metal infiltration and
diffusion bonding also being employed for the latter. For very thin layers
of ceramic or metal, vapor deposition processes including ion plating
and/or plasma spraying have been employed.
Laminar composites incorporating glass layers can be bonded by hot pressing
or glass sintering at temperatures where the glass can wet adjoining
materials. In fact, some composites include powdered glass simply as a
sealing material to bond otherwise incompatible metallic or ceramic
laminae into a unitary structure. U.S. Pat. Nos. 4,868,711 (Hirama et al.)
and 3,490,887 (Herczog et al.) offer examples of glass-containing
composites.
In the prior art, ceramic/metal laminates have quite often used relatively
thick ceramic laminae. For example, Cao and Evans, in "On Crack Extension
in Ductile/Brittle Laminates", Acta metall. mater., Vol. 39, No. 12 pp.
2997-3005 (December 1991), have described alumina/aluminum composites
comprising 1 mm commercial IC substrate alumina sheets as ceramic layers.
Thinner ceramic layers have been produced in ceramic capacitors, electronic
substrates, and similar composites by co-sintering. For example, Marshall
et al. in "Enhanced Fracture Toughness in Layered Microcomposites of
Ce-ZrO2 and Al2O3", J. Am. Ceram. Soc., 74 [12] 2979-87 (1991), describe
laminar composites of Ce-ZrO2 and Ce-ZrO2+Al2O3 with layers on the order
of 10-100 micrometers in thickness using colloids with sequential
centrifugation and co-sintering.
Unfortunately, ceramic/ceramic and ceramic/metal co-sintering processes
present serious obstacles to the attainment of truly homogenous and
defect-free composite structures. Such obstacles include relatively crude
and non-uniform layer structures, curling or wrinkling of the structures
during the co-sintering process, and pin-hole or other layering defects.
In the case of metal/ceramic composites, for example, the metal powder and
the ceramic powder must sinter in nearly the same temperature range, and
the densification shrinkages of the ceramic and metal cannot be too
different. Also, kinetically and thermodynamically stable combinations of
materials should be used, with compatible partial pressures of gaseous
species (PO2 and PH2O) being maintained during sintering.
The manufacture of metal/ceramic composites for capacitors provides a
practical example of co-sintering as presently practiced. In that process,
ceramic powders and metallic powders are combined, eg., by tape casting
the ceramic layer and screen printing the metal layer, and the resulting
laminae are stacked and heated to remove binders and solvents. Thereafter,
the debindered stacked structures are heated to sinter the metal and
ceramic materials. However, before sintering is attempted the stacks are
first diced into relatively small chip sizes as required for capacitor
use.
While satisfactory for capacitor fabrication, the manufacture of
mechanically durable composites from continuous sheets of unsintered
powder materials is not practical. This is primarily due to the very high
shrinkage factors involved. Even with relatively well matched ceramic and
metallic starting materials, extensive layer shrinkage during powder
sintering favors the formation of multiple layer defects, eg., pin-hole
defects as in metal capacitor films. In addition, large scale layer
fluctuations and laminate distortions can occur, as previously noted.
These kinds of defects are not significant in products such as capacitors,
which are generally below 1 cm size and which can tolerate a relatively
high number of process-induced pin-hole defects in the metal layers. Where
the mechanical properties of the composites are key, however, such defects
are not acceptable.
It is therefore a principal object of the present invention to provide an
improved process for the fabrication of microlaminated composites which
provides well-bonded laminar articles with improved layer structure.
It is a further object of the invention to provide microlaminated
composites offering improved physical integrity and structural uniformity.
Other objects and advantages of the invention will become apparent from the
following description thereof.
SUMMARY OF THE INVENTION
The present invention provides microlaminated composites in a wide variety
of brittle/ductile, brittle/semi-brittle and brittle/brittle combinations.
The brittle layers in these composites are introduced into the composite
structure as highly uniform, pre-sintered ceramic sheets or tapes. The
ceramic sheets or tapes are provided as flexible, high-strength
polycrystalline ceramic foils, these foils having thicknesses generally
not exceeding about 250 micrometers. The foils may be of near-theoretical
density (non-porous), or they may be porous (up to about 60% porosity),
but in all cases they will have been sintered prior to use to
approximately the level of density selected for the composite structure to
be provided.
The composites of the invention are well-bonded laminar structures wherein
the ceramic foils are bonded directly to ductile, semibrittle or brittle
substrate materials. Hence the foil-substrate interfaces are essentially
free of added adhesive interlayer materials. The composites may be
fabricated by low-pressure, high temperature consolidation methods, and
exhibit excellent ceramic/substrate bonding even at the low consolidation
pressures customarily employed.
The bonding achieved is attributed to the uniform nature and unique high
temperature bonding characteristics of the flexible polycrystalline
ceramic foils forming the brittle layer materials in these composites.
This bonding behavior, attributed in part to plastic or superplastic
deformation of the ceramic at elevated temperatures, provides firm
adhesion to metallic, intermetallic and other ceramic laminae in the
composite even where extremely low-pressure lamination processing is used.
In one aspect, then, the invention resides in an improved method for making
microlaminated composite articles incorporating ceramic layers. In
accordance with that method, at least one sheet of flexible sintered
crystalline ceramic foil and at least one inorganic substrate layer of a
metal, intermetallic compound or crystalline ceramic material are
provided. The ceramic foil sheet is stacked on the substrate layer, and
the resulting stack is heated under pressure at a temperature and for a
time at least sufficient to obtain permanent bonding of the ceramic foil
sheet to the inorganic substrate layer.
While in concept the substrate layer could comprise the surface of a bulk
metal, intermetallic, or ceramic object, it is more typically a thin sheet
of substrate material. Hence, the composite is generally made by providing
a plurality of sheets of flexible sintered crystalline ceramic foil and a
plurality of the inorganic substrate layers, the latter also being in the
form of preconsolidated sheets (sheets essentially free of organic binder
constituents).
The two types of sheets are formed into a sheet stack wherein at least some
of the sheets of ceramic foil are in contact with adjacent inorganic
substrate sheets. In the preferred stack configuration, the ceramic foil
sheets are interleaved with the substrate sheets, most preferably in an
alternating layer configuration, although other stacking or weaving
configurations may alternatively be used.
The resulting sheet stack is then heated as a unit, with the application of
light pressure, to the temperature required to obtain the desired
permanent bonding of the ceramic foil sheets to the substrate sheets. The
pressures required during heating can be as small as 1 kPa and will seldom
exceed 500 kPa, although much higher pressures can be used if for some
reason they are deemed desirable.
The temperatures needed to obtain permanent bonding of these ceramic foils
to inorganic substrates are generally below the lowest melting temperature
of the ceramic foils and inorganic substrate materials used in the
composite. By the lowest melting temperature is meant the lower of the
respective melting temperatures of the ceramic foil and substrate layer
material used. This permits bonding to be carried out without any gross
deformation of either of the layered materials.
The minimum temperature for good bonding is presently considered to be a
temperature approximately 300.degree. C. below the lowest sintering
temperature of the ceramic foil and inorganic substrate layer(s). Again,
by the lowest sintering temperature is meant the temperature corresponding
to the lower of the respective minimum sintering temperatures of the
ceramic foil and substrate layer material used.
As previously noted, the preferred practice in accordance with the
invention is to provide multiple inorganic substrate sheets, these sheets
most preferably consisting of sheets of metallic or intermetallic foil.
These foil sheets are partly or wholly interleaved with the ceramic foil
sheets to provide a sheet stack of alternating layer structure, or of more
complex layered structure, if desired. Of course, more than one type of
ceramic foil and one type of substrate layer material can also be used.
In another aspect the invention includes microlaminated composite articles
of improved structural homogenity and integrity. These articles generally
comprise a plurality of layers (two or more) of ceramic foil, these foil
layers being partly or wholly interleaved by a plurality of metallic or
intermetallic substrate layers. The layers of ceramic foil are directly
bonded to the substrate layers in contact therewith, direct bonding
meaning ceramic/metallic bonding between the ceramic foil and metal,
without the aid of any intermediary cementing or sealing material.
Characteristic of these composites, the layers of ceramic foil and the
metallic substrate layers will have thicknesses not exceeding about 250
micrometers (more preferably not exceeding 100 micrometers), while the
metal foil layers will have thicknesses up to about 1 mm. In addition, the
ceramic and metal layers will have at least one dimension of length or
breadth in excess of 1 cm, yet will be essentially free of pin-hole
defects. This excellent layer quality is due to the fact that only
presintered sheets and foils are used in the fabrication of the
composites. Thus the shrinkage encountered in co-sintering processes,
which invariably tends to introduce defects into the ceramic or metallic
layers of co-sintered structures, is entirely avoided.
In addition to their excellent structural homogeneity and integrity, the
microlaminated composites of the invention offer physical properties which
are closely controllable over a very broad range of permissible values.
Hence, the properties of the ultimate products depend simply upon the
compositions, thicknesses, and properties of the ceramic foils and
metallic or intermetallic materials selected for incorporation therein.
Numerous applications for microlaminated composites provided in accordance
with the invention have been identified. Depending upon whether brittle,
semi-brittle, or ductile interleaving materials are used to construct the
composites, structural microlaminates for cutting tool applications, for
high temperature components for heat engines, for refractory support
structures in catalytic conversion applications, and for a wide variety of
other products requiring physical properties not available in components
formed of only a single ceramic or metallic material, have now been found
to be practicable.
DESCRIPTION OF THE DRAWING
The invention may be further understood by reference to the drawings
wherein:
FIG. 1 is an optical photomicrograph of a cross-section of a
brittle/ductile microlaminated composite of the invention;
FIG. 2 is an optical photomicrograph of an edge-ground cross-section of a
brittle/semibrittle microlaminated composite of the invention;
FIG. 3 is an optical photomicrograph of a cross-section of a
brittle/brittle microlaminated composite provided in accordance with the
invention;
FIG. 4 schematically illustrates a first alternative construction for a
composite sheet stack useful in accordance with the invention; and
FIG. 5 schematically illustrates a second alternative construction for a
composite sheet stack useful in accordance with the invention.
DETAILED DESCRIPTION
A particular advantage of microlaminate manufacture in accordance with the
invention is that the usual sequence of fabrication, i.e., ceramic
densification after formation of the laminate with green (unsintered)
ceramic material, is reversed. Thus sintering of the ceramic laminae
precedes microlaminate fabrication, permitting full non-destructive
inspection of the ceramic, metallic, or other laminae by optical, X-ray,
or other methods prior to composite fabrication,
This construction sequence not only insures much higher product
reliability, but also greater latitude in the design of the layer
structure of the composite. Hence, as dense ceramic sheets of different
sizes and compositions are easy to make, sort and store, complicated
structures can be made by laying up ceramic sheets of varying sizes and
compositions. The sheets need not run across the entire structure.
Based on this approach to microlaminate construction, microlaminates
incorporating substantially defect-free layers, yet having layer sizes in
one or more dimensions easily exceeding one centimeter, or even 5 or 10
centimeters, can easily be made. And the construction is not limited to
extremely thin metal films, or to metals with expansion properties closely
matching the properties of the ceramic components of the composites.
A wide variety of polycrystalline ceramic materials may be used to provide
thin flexible ceramic foils for the construction of microlaminated
composites as herein described. Methods for making flexible ceramic foils
are disclosed in U.S. Pat. No. 5,089,455, that patent disclosing that many
different ceramics are amenable to sintered sheet forming in accordance
with that patent.
Examples of ceramics useful for foil fabrication include zirconia,
stabilized or partially stabilized zirconia, hafnia, alumina,
.beta.-alumina, .beta.''-alumina, silica, titania, mullite, spinel,
chromium oxide, sialon, hasicon, silicon or titanium carbides and/or
nitrides, zircon, zirconium carbide, and titanium diboride. A variety of
stabilizers may be present in the zirconia-based ceramics, including any
of the well-known alkaline earth oxide and rare earth oxide stabilizers
alone or in combination.
In addition to the other single-phase ceramic systems of the above types,
two- or multi-phase ceramics containing the above materials in any of a
wide variety of combinations may also be used to provide sintered flexible
sheet. Ceramic foils consisting essentially or at least predominantly
(greater than 50% by weight) of the above ceramic materials or mixtures
thereof are particularly preferred.
The foil may also be produced through the crystallization of a glass in
powder or thin sheet form, ie., it may be a glass-ceramic sheet. Examples
of glass-ceramics amenable to sheet-forming and use as ceramic foils in
accordance with the invention particularly include alkaline earth
aluminosilicate glass-ceramics, such as cordierite and anorthite
glass-ceramics.
Flexible ceramic foils such as above described are uniquely suited for
microlaminate fabrication because of their strength and toughness, as well
as because of their flexibility. To best facilitate handling of the foils,
and to gain full advantage from the use of pre-sintered ceramic foils in
the composite structure, we prefer to use ceramic foils having a toughness
(K.sub.IC) of over 1.5 MPa .sqroot.m, more preferably over 2.0 MPa
.sqroot.m.
The thickness of the flexible ceramic foils to be used for microlaminate
fabrication in accordance with the invention significantly affects the
lamination performance of the material. Sheet thicknesses in excess of 500
micrometers are to be avoided, while sheets on the order of 250
micrometers or less in thickness should offer adequate lamination
performance. Foils thinner than about 1 micrometer offer no advantage and
require that a large number of sheets be handled. Thus we generally prefer
to employ foils with thicknesses in the 10-100 micrometer size range, with
25-50 micrometer foils being particularly preferred for best composite
bonding.
In general, thicker sintered sheet can be substituted for thinner sheet
where higher pressure consolidation of the sheet stack can be tolerated.
On the other hand, the preferred thinner sheet is easier to bend
elastically, develops higher stress for a given applied pressure,
plastically deforms more rapidly by high temperature creep, and thus bonds
faster. Also, thinner ceramics develop more interfaces, which can lead to
higher toughness. Advantageously, flexible sintered sheet of the kinds
provided in accordance with the aforementioned patent exhibit excellent
flatness, such that very little plastic flow and deformation of the sheet
is required to form a good bond against adjacent material layers such as
metal foils.
A wide variety of substrate or interlayer materials can be combined with
the described flexible sintered ceramics to produce well bonded
composites. Such materials may include metals (single-component as well as
metal alloys), intermetallics, and other ceramics. Examples of metals
useful for such microlaminates are ferrous metals including stainless
steels, copper, aluminum, nickel, nickel-based alloys, titanium,
molybdenum, niobium, tungsten, tantalum, chromium, and high-temperature
metal alloys such as the chromium-based alloys. Representative examples of
intermetallic compounds which may be used include titanium aluminide and
nickel aluminide.
Ceramic interlayer materials, including any of the ceramics useful for the
flexible sintered ceramic foils themselves (but differing in composition
therefrom to achieve a composite material with composite properties), can
also provide integral and well-bonded composites. Other potential ceramic
interlayer materials could include thin sheets or tapes of single crystal
materials, such as sapphire or silicon. The ranges of thickness and
preferred thickness applicable to the selection of ceramic foils as above
described are also applicable to the selection of substrate sheets, for
composites incorporating a plurality of substrate layers.
Due to the thinness of the laminae present in the composites of the
invention, we have found that integral composite structures can be
fabricated even where there is a substantial difference in thermal
expansion coefficients between the layered materials. The use of such
diverse materials as metals and ceramics may be facilitated through the
use of flexible ceramics of lower elastic modulus, higher porosity, and
smaller grain size, leading to a lower deformation temperature for the
ceramic sheet. In the case of high expansion metal interlayers, a lower
elastic modulus, greater porosity, and/or higher ductility in the metal
all promote better structural integrity in the product.
In general, the highest degrees of interface bonding and structural
integrity have been observed in microlaminates of brittle/ductile
(ceramic/metallic) type. Good bonding is also achievable in intermetallic
and all-ceramic microlaminates, but is presently considered to require
closer attention to interfacial geometry, microstructure and chemistry in
order to achieve optimum results.
As previously indicated, the levels of pressure required to achieve well
bonded composites in these microlaminate systems is extremely low.
Pressures as low as 1 kPa have been used successfully in some of the
preferred metal/ceramic systems, with pressures above 50 kPa seldom being
required in these systems except where sheets exhibiting relatively poor
flatness and/or relatively high thickness are employed.
As previously noted, the lamination temperatures required for good bonding
in these composites are relatively low, with temperatures will below the
minimum sintering temperatures of the ceramic foil and metal substrate
layers being useful for this purpose. Preferably, however, lamination will
be carried out at a lamination temperature T.sub.L within a preferred
range of lamination temperatures as follows:
(T.sub.LS -200)<T.sub.L <T.sub.LM
wherein TLS and TLM are the lowest sintering and lowest melting
temperatures, respectively, of the ceramic foil and metal substrate layers
included in the composite structure.
Advantageously, due to the extremely low consolidation pressures which can
be used, expensive hot pressing equipment is not required, and
consolidation tooling for these microlaminates can be made of inexpensive
and easily machinable materials of only moderate strength. In fact, simple
porous refractory oxides can be used for molding surfaces, these providing
both adequate strength and sufficient high temperature stability for even
the most refractory of the available flexible ceramic sheet materials.
A further advantage stemming from the high flexibility of the component
materials used in these microlaminates is that the near-net-shape
fabrication of composite products becomes much more practical. Thus, even
for complicated shapes, the component sheets can be laid up at room
temperature in substantially the final configuration of the desired
product, and consolidation of the microlaminate thereafter achieved at
moderate pressure and without the large shrinkage and/or shape changes
normally accompanying the consolidation of fiber-reinforced composites by
high pressure processing. Additionally, since the densification steps
precede and are separate from the subsequent shape-forming and bonding
steps in our method, the precise construction of microlaminates with
graded or other complex composition profiles becomes achievable.
In actual practice, of course, there is a practical limit to the strain
which can be applied at room temperature without breakage of ceramic foils
such as described, and this limit can be exceeded where complex
three-dimensional curves are to be developed in the microlaminated
composite products to be fabricated. A simple example of such a case would
be a domed or saddle-shaped microlaminated structure with curvature about
two or more axes. For shapes of this complexity, large amounts of plastic
deformation (>10%) are required, not for the lamination, but to form the
shape.
Whereas metals, polymers, and the like can easily be corrugated or
otherwise shaped, the shaping of essentially flat sintered ceramics has up
to now not been practical. Instead, the complex forming of ceramic bodies
had been achieved by direct casting or by the shaping of green ceramic
material disposed on a polymer or paper support, the shaped green material
then being stacked and fired.
Unexpectedly we have now discovered that, at appropriate temperatures,
these ceramic foils can be rapidly reformed with little or no change in
physical properties, after sintering but prior to laminating, to provide
corrugated or three-dimensionally curved thin foil shapes. Thus these
ceramic foils can, if desired, be preformed into complex curves prior to
stacking, and foils of matching curvature can then be stacked and quickly
consolidated to provide microlaminated composites of complex
three-dimensional shape as efficiently as for flat or cylindrical
composites. This near-net--shape preforming approach both reduces the
plastic deformation necessary during lamination and enhances the ease of
bonding.
The process of providing curved preformed ceramic foils as described
comprises heating a flexible ceramic foil to a temperature which is in the
range from about 300.degree. C. (more preferably 200.degree. C.) below the
minimum sintering temperature of the ceramic to just below the melting
temperature thereof, and then applying pressure to the foil sufficient to
shape it into a selected curvature at a rapid strain rate. By a rapid
strain rate is meant a strain rate above about 5.times.10.sup.-3 /second,
preferably above 10.sup.-2 /second. Only very slight pressures are
required for this purpose.
This strain rate is a rate well in excess of conventional reforming strain
rates for polycrystalline ceramic sheet or plate, and is a commercially
viable rate. In fact, we have achieved strain rates in the range of
1.5-5.times.10.sup.-2 /second for both foil crimping and for foil
elongation, with ceramic foil of approximately 20 micrometer thickness as
above described.
Strain rates of this magnitude readily permit the production, for example,
of corrugated ceramic foil at a corrugation radius of curvature of about
0.328 mm (3.05% permanent strain) in a foil reforming interval of about
0.75 seconds. Such strain rates are considered to be attainable in the
above-disclosed temperature range, for any of the aforementioned ceramic
foils, at foil thicknesses up to about 250 micrometers.
For complex bodies, then, a preferred method of fabrication is to produce
individual preformed ceramic and metal foil sheets of the appropriate
shape through plastic or superplastic deformation as described, stack them
at room temperature, and then laminate them. The result is a substantial
reduction in the amount of material transport necessary during formation
of complex parts, ie., reduced lamination time.
This preferred approach also provides a "decoupling" of the steps of
composite fabrication, in that the shaping of the various metallic and
ceramic foil materials for a selected composite structure can be carried
out at temperatures and pressures optimum for the individual layers. Thus
the layers do not all have to be taken through the entire fabrication
process together, but may be individually shaped, coated, or otherwise
pretreated.
Techniques analogous to those used for hot glass sheet forming may be used
for ceramic foil preforming in accordance with the above process. These
include the use of preheated die or mold sets of refractory ceramic (eg.,
a set of corrugated alumina molds), refractory forming rolls or gears, or
vacuum or pressure forming against a porous or non-porous forming mold.
Due to the slight thicknesses of these ceramic foils, extremely rapid
heating rates conducive to rapid reforming can be used without risking
thermal shock damage.
The efficiency of ceramic sheet reforming as above described is
demonstrated by the case of stabilized zirconia sheet (zirconia plus 2
mole percent of yttria stabilizer), provided in the form of flat, thin,
flexible ceramic foil ribbons of 13-28 micrometers thickness. A matched
set of refractory (alumina) corrugated reforming dies was preheated in a
furnace to approximately 1370.degree. C., the furnace then opened, and the
flat foils placed between the molds. The furnace was then closed and
brought back to temperature, at which time the dies were closed with an
applied reforming pressure of 740 Pa (0.1 psi) for a few seconds. The
products removed from between the die set were corrugated zirconia ribbons
(corrugation period of 4.8 mm and amplitude of 1.2 mm), all of which
retained excellent flexibility in directions not constrained by the
corrugation.
For most successful fabrication of microlaminates by low-pressure
consolidation as described, a number of useful guidelines have been
developed. These include (i) carrying out sheet stack consolidation at a
high enough temperature that all of the materials employed can plastically
deform; (ii) selecting constituent materials such that the thermal
expansion coefficients are not too different; (iii) positioning the more
brittle laminae (usually the ceramic foils) so that they are in
compression after lamination at ambient temperature; (iv) selecting
constituent materials which are kinetically and thermodynamically stable
with respect to one another (and with respect to the bonding environment)
at bonding temperatures; (v) selecting the flattest available ceramic foil
materials; and (vi) avoiding unnecessary thickness variations in the
ceramic and substrate layers (except where thickness variations are part
of the composite design).
From the standpoint of consolidation processing, a consolidation approach
wherein one or more of the molds used to apply consolidation pressure to
the sheet stacks is free to change orientation and/or shape to apply the
most uniform possible pressure to the stack is useful. The employment of
mold release agents which are compatible with the mold and sheet stack
materials at the selected molding temperatures will be found beneficial in
some cases. Finally, it is particularly desirable to avoid the
introduction of foreign matter into the sheet stack, since such can
introduce defects even where the sheets have previously been consolidated
to defect-free condition.
The invention may be further understood by reference to the following
detailed examples thereof, which however are intended to be illustrative
rather than limiting.
EXAMPLES 1-13
To provide a series of brittle/ductile (ceramic/metal) microlaminated
composites, thin dense flexible ceramic foils are first provided by a
flexible sheet manufacturing process. The process used is as described in
U. S. Pat. No. 5,089,455 (Examples 1-3), utilizing commercially available
zirconia and alumina powders. The zirconia powders used were commercial
yttria-stabilized zirconia powders TZ-2Y and TZ-3Y, purchased from the
Tosoh Chemical Company of Tokyo, Japan, while the alumina powder used was
powder RC-HP-DBM alumina (MgO free) obtained from Malakoff Industries,
Malakoff, Tex., U.S.A.
Two powder batches were used in the preparation of the foils. The first
consisted of TZ-3Y yttria-stabilized zirconia (containing 3 mole percent
Y.sub.2 O.sub.3). The second consisted of a mixture of alumina (20 weight
percent) and TZ-2Y yttria-stabilized zirconia (80 weight percent), the
latter containing 2 mole percent of yttria as a stabilizer.
Three different peak firing temperatures (1300.degree. C., 1350.degree. C.,
and 1430.degree. C.) are used to prepare a total of four different ceramic
foil types from these powders, the firing temperature variations being
used in order to provide a range of different grain sizes in the ceramic
materials. The samples are in each case maintained at the peak firing
temperature for two hours in an air atmosphere. All of the resulting sheet
samples are essentially pin-hole-free square sheets 20 mm on a side and
having a sheet thickness in the range of 30 to 45 micrometers (<2 mils).
Sheets of second layer materials for the microlaminates to be provided are
also provided, these consisting of sections of essentially pin-hole-free
metal foil having thicknesses of 51 or 38 micrometers (2 or 1.5 mils). The
foils are composed either of 410 stainless steel or of nickel (99.5%
weight purity). Table 1 below sets forth a summary of the ceramic foils
and metal foils used for microlaminate fabrication, including
compositions, thicknesses and, where appropriate, sintering temperatures
for each of the sheets and foils selected.
TABLE 1
______________________________________
Sheet Sheet Thickness Sintering
I.D. Composition (.mu.m) Temp.(.degree.C.)
______________________________________
A zirconia (3 m % Y.sub.2 O.sub.3)
30-45 1430.degree.
B zirconia (3 m % Y.sub.2 O.sub.3)
30-45 1350.degree.
C zirconia (3 m % Y.sub.2 O.sub.3)
30-45 1300.degree.
D 20% wt. alumina +
30-45 1300.degree.
80% wt. zirconia
(2 m % Y.sub.2 O.sub.3)
X nickel (99.5%) 51 --
Y 410 stainless steel
38 --
Z 410 stainless steel
51 --
______________________________________
To form sheet stacks for microlaminates from the materials shown in Table
1, the metal foils are cut into 20 mm (0.75 in) squares and layered in
alternating fashion with the squares of presintered ceramic foil. The
sheet stacks are then sandwiched between alumina pressing surfaces
(consisting of flat alumulna platens or, for curved products, curved
alumina molds made from alumina crucible sections), and a light force (on
the order of 7.6 kPa) is then applied to the selected pressing surfaces by
means of a weighted alumina crucible.
The sheet stacks so assembled and compressed are next heated under the
applied pressing forces to a peak bonding temperature in the range of
1300.degree.-1400.degree. C., this peak temperature being maintained for
about 2 hours. Heating is carried out in a tungsten mesh vacuum furnace at
a pressure of about 10.sup.-5 tort, with heating and cooling rates in the
range of 650.degree.-700.degree. C. per hour being used.
A summary of the microlaminated composites made by the above procedure is
set forth in Table 2 below. Included in Table 2 for each of the Examples
reported are a sample number, sheet identifications for the ceramic foil
and metal foil used (from Table 1), and the number of layers, bonding
temperatures and bonding pressures used for each sample.
TABLE 2
______________________________________
Microlaminated Composites
Example Ceramic Metal No. of
Applied Bonding
No. Sheet Foil Layers
Press.(kPa)
Temp.(.degree.C.)
______________________________________
l A Z 3 2.1 1300.degree.
2 A Z 9 2.1 1300.degree.
3 A Z 7 2.8 1350.degree.
4 A Z 7 2.8 1400.degree.
5 A Y 7 5.5 1350.degree.
6 A Y 27 7.6 1400.degree.
7 B Y 31 7.5 1400.degree.
8 C Y 25 7.6 1400.degree.
9 C Z 21 8.3 1400.degree.
10 A X 13 8.3 1300.degree.
11 A X 13 8.3 1350.degree.
12 C Z 7 8.3 1400.degree.
13 D Z 25 8.3 1400.degree.
______________________________________
Some procedural variations in the preparation of the samples in Table 2 may
be noted. Examples 1-8 in the Table were consolidated without any side
constraints, whereas the sheet stacks of Examples 9-11 were prevented from
shifting during firing by a square jig of sintered alumina. In the
manufacture of curved Example 12, the curved crucible sections used to
compress the sheet stack were not of identical curvature, but instead
displayed a center gap when in contact with each other. The pieces used to
press curved Example 13 were of closely matched curvature.
Evaluation of the microlaminates provided in accordance with these examples
included a determination of the strength of the interlayer bonding
achieved. One severe test employed to test for good bonding was edge
cutting and edge grinding, a procedure which tended to initiate edge
delamination in composites with somewhat lower interlayer bond strength.
Electrical resistivity measurements were also performed.
An inspection of the Examples listed in Table 1 indicated at least some
strong interlayer bonding in every case, with the extent and/or strength
of the bonding generally increasing in the stainless steel samples at the
higher bonding temperatures and pressures. Thus while Example 1 was well
bonded in spots, Examples 6-9 demonstrated very good bonding across most
or all of the samples, except in areas where insufficient pressure had
been applied. Examples 8 and 9 utilized zirconia sheet sintered at
1300.degree. C. for smaller crystal grain size; this could have enhanced
the high temperature plasticity and thus the deformation bonding of the
laminae. The nickel-containing samples (Examples 10 and 11) also showed
good interlaminar bonding, with slightly better integrity being shown by
Example 10 which was bonded at the lower of the two hot-pressing
temperatures used.
Curved samples 12 and 13 each had a radius of curvature somewhat less that
3.5 cm. Example 12 was well bonded at both edges but not at the center,
due to the center gap in the molding pieces. However, Example 13 was
excellently bonded throughout, a result we attribute in part to the use of
a zirconia-alumina ceramic laminae which had been pre-sintered by firing
at 1300.degree. C. The combined effect of the alumina second phase and the
low sintering temperature yielded a fine grain size (below 0.5
micrometers) in this sheet, which facilitated high temperature plastic
deformation of the ceramic during consolidation of the sheet stacks.
Several of the microlaminated composites reported in Table 2 were tested
for room temperature resistance both parallel and perpendicular to the
plane of the microlaminates. Table 3 below reports the results of these
measurements for selected samples.
TABLE 3
______________________________________
Electrical Resistivity
Perpendicular
Parallel
Example No. Resistance (ohms)
Resistance (ohms)
______________________________________
6 1.0 .times. 10.sup.6
0.05
7 2.5 .times. 10.sup.6
0.05
8 4.0 .times. 10.sup.6
0.08
13 20 .times. 10.sup.6
0.1
______________________________________
The results shown in Table 3 are representative of the highly anisotropic
nature of these ceramic/metal microlaminates in terms of both composition
and physical properties. Based on these results, other chemical and
physical properties of these composites are also expected to be highly
anisotropic.
In utilizing flexible ceramic foils of fine grain size to provide
microlaminated products such as described, it is found that long heating
times are not required to produce excellent interlayer bonding of the
composites. Lamination trials using flexible stabilized zirconia (2%m
Y.sub.2 O.sub.3) sheets of 40-45 micrometers thickness, interlaminated
with 410 stainless steel foil of 51 micrometers thickness and hot pressed
at 16.5 kPa (2.4 psi) and 1400.degree., provided well-bonded
microlaminates in firing times of 30 minutes, 10 minutes, and even one
minute. While it was found useful at these temperatures and pressures to
use molybdenum foil separation sheets to prevent adherence of the
composites to the alumina molding platens, the bonding results clearly
demonstrate that very rapid forming operations, e.g., rolling, could be
successfully applied to the microlamination process.
FIG. 1 of the drawing is an electron photomicrograph of a cross-section of
a stainless steel/zirconia (TZ-2Y) microlaminate such as reported in Table
1, wherein the white bar represents a dimension of about 100 .mu.m. That
figure shows a view of a cut and ground edge of the composite, the
composite having been made using 50 .mu.m steel foil and 50 .mu.m ceramic
foil by lamination for 2 hours at 1300 .degree. C. and a pressure of 17.2
kPa. The high degree of uniformity of the layers and the absence of layer
defects and interlayer edge delamination are plainly evident from this
photomicrograph.
While the bonding process operative to produce durable microlaminates in
accordance with the invention is not fully understood, it is presently
theorized that high temperature deformation of the flexible sintered
ceramic layers is the principal bonding mechanism. This deformation may
occur through grain boundary diffusion controlled creep (Coble creep) or
by lattice diffusion controlled creep (Nabafro-Herring creep). Such creep
is virtually certain to be present during plastic deformation of these
materials at high temperature, along with grain sliding and grain
switching events when the plastic deformation reaches high strains.
As previously indicated, the method of the invention is not limited to the
production of microlaminates wherein all layers are of the same size, or
of alternating compositions, or of only two different compositions.
Instead, the ability to use pre-sintered ceramic and metal foil permits
the design and production of much more elaborate layered constructions
offering unique properties, as illustrated by the following examples.
EXAMPLE 14
Three strips of flexible sintered ceramic foil composed of stabilized
zirconia (2 mole % yttria), each strip being 6.4 mm (1/4 in.) wide, 36.5
mm (11/4 in.) long, and 40-45 micrometers thick were provided for
incorporation in a sheet stack. To form the base layer in the stack, these
strips were alternated side-by-side with two strips of 410 stainless steel
foil of approximately the same dimensions as the ceramic strips.
The next layer of the stack was similarly formed, but with three strips of
metal foil alternated with two strips of ceramic, and with the
side-by-side strips of ceramic and metal foil rotated 90.degree. from the
alignment of the strips in the preceding layer. Square leaves of the steel
foil were then placed on the top and bottom of the two alternating strip
layers.
FIG. 4 of the drawing provides a schematic illustration in the form of an
exploded view of this layer construction, showing the side-by-side
orientation of the ceramic foil strips 12 and steel foil strips 14 forming
the alternating strip layers. The layers thus formed are then positioned
between the outer steel foil leaves 16.
To form a sheet stack for a final composite plate, the above layer
construction was repeated two more times, and the resulting sheet stack
was then consolidated at 1400.degree. C. at a pressure of 4.1 kPa (0.6
psi) for two hours. Inspection of the composite thus provided indicated
that good interlayer bonding among all of the layers of the composite had
been achieved.
A particular advantage of the composite structure described is that the
ceramic laminae effectively form a creep resistant skeleton, or matrix,
within which the metal can provide ductility in all fracture directions as
well as a continuous electrically conducting network. Additionally, the
use of ceramic strips, rather than sheets as in the previous examples, can
provide products of essentially unlimited size, while the use of
wedge-shaped or other irregular strip configurations could provide
products of even more complex curvature and/or shape.
Other complex sheet stack configurations, including woven configurations,
have also been demonstrated. A schematic illustration of a typical woven
structure utilizing a simple weaving design is provided in FIG. 5 of the
drawing. In that construction, alternating strips of ceramic foil (light
strips, strips 12 being representative) and metal foil (hatched strips,
strips 14 being representative) have been interwoven with a similar number
of crossing strips, also of ceramic and metal foil. An example of the
processing of a microlaminated composite constituting a simple variant of
the above design is provided by the following example.
EXAMPLE 15
Four strips of sintered zirconia (2 mole % yttria) sheet, approximately 40
micrometers thick by 38.1M (11/2 in.) long by 4.8 mm (3/16 in.) wide were
woven in a simple basket weave with six strips of 410 stainless steel
foil. The steel foil was approximately 51 micrometers thick, 38.1 mm (11/2
in.) long, and 4.8 mm (3/16 in.) wide. This woven structure was placed on
a base layer comprising a 38.1 mm square zirconia sheet sandwiched between
facing layers of steel foil of the same square dimensions. This multilayer
structure was duplicated once, and then a top layer consisting of the
zirconia-sandwiched steel foil was finally added. The sheet stack with
woven laminae thus provided was then hot pressed at 4.1 kPa (0.6 psi) and
1400.degree. C. in a vacuum for two hours. Inspection of the resulting
microlaminated composite indicated excellent interlaminar bonding,
although with slight cracking of the composite at points of
ceramic/ceramic overlap. It was considered that a process change wherein
the application of pressure to the sheet stack would be delayed until
after heating to the peak bonding temperature would easily avoid these
defects.
The advantages of this weaving approach are several. First, it can overcome
delamination difficulties in microlaminate composite systems wherein
interlayer bonding is limited by slight interlayer bonding
incompatibilities. Secondly, woven layers permit the fabrication of larger
and more complicated shapes structures because woven sheets can more
readily be reshaped into curved configurations.
The use of porous pre-sintered ceramic foil as well as non-porous or highly
densified sintered sheet can also yield useful microlaminated composites,
as shown by the following example.
EXAMPLE 16
Six sheets of partially stabilized zirconia (zirconia-3 mole % yttria)
having a 22.2 mm (7/8 in,) square shape and a thickness of about 60
micrometers are interlaminated between seven sheets of 410 stainless steel
foil. In this case, however, the zirconia sheet is pre-sintered sheet
having a porosity of about 50 volume percent, the sheet having been
produced by the firing of green zirconia sheet to a peak temperature of
only about 1100.degree. C. for 2 hours. This sintering treatment removes
all organic and other contaminants from the zirconia but does not fully
consolidate the zirconia powder to zero open porosity.
The sheet stack thus provided is hot-pressed at 1200 .degree. C. and at a
pressure of 12.1 kPa (1.75 psi) under vacuum for 2 hours. Inspection of
the resulting microlaminate indicated that a well-bonded composite article
had been achieved. The overall porosity of the final microlaminate
(zirconia plus steel) was measured at about 25%, indicating that
consolidation without a reduction in the porosity of the ceramic foil had
successfully been achieved.
Another advantage of the method of the invention resides in the ease with
which the relative proportions of brittle and non-brittle materials may be
adjusted, or changed in specified target areas with these composites,
without significant impact on the process of composite consolidation. The
following examples illustrate these features.
EXAMPLE 17
A microlaminated composite comprising a relatively low volume fraction of
polycrystalline ceramic is fabricated from a predominantly steel sheet
stack, the stack being made by alternating 410 stainless steel layers 51
micrometers (2 mils) thick with layers of zirconia - 2 mole % yttria
sintered sheet less than 15 micrometers thick. To obtain a high ductile
phase content, the sheet stack was constructed by stacking three steel
layers on a single ceramic foil layer, repeating this layer group two
additional times and covering with a single ceramic foil layer. This core
stack was then sandwiched between steel covering laminae, each being two
steel foil layers in thickness.
This stack was then hot-pressed at 1400.degree. C. for two hours in a
vacuum furnace under an applied pressure of less than 13.8 kPa (2 psi).
The resulting composite, containing only approximately 15.5 volume % of
zirconia ceramic, was well bonded and durable.
EXAMPLE 18
A microlaminated composite comprising a relatively low volume fraction of
ductile metal is fabricated from a predominantly ceramic sheet stack, the
stack being made by placing one 410 stainless steel foil layer (51
micrometers thick) between two three-sheet groups of flexible sintered
zirconia ceramic (zirconia - 2 mole % yttria) foil. The zirconia foils
forming the outer groups were approximately 30-35 micrometers in
thickness, yielding a group thickness of about 100 micrometers.
The resulting sheet stack was consolidated by hot pressing at 1400.degree.
C. for 2 hours at a pressure slightly less than 13.8 kPa (2 psi) in a
vacuum furnace. The consolidated product was a well-bonded and durable
microlaminate with a metal content on the order of 20 percent by volume.
EXAMPLE 19
An asymmetric microlaminated body was made by alternating three zirconia -
2 mole % yttria ceramic layers about 30-35 micrometers in thickness with
one layer of stainless steel foil about 51 micrometers (0.02 in) in
thickness, repeating this layering pattern once. The resulting sheet stack
was consolidated by hot-pressing at 1400.degree. C. for 2 hours at a
pressure below 13.8 kPa (2 psi) in a vacuum furnace. The product was a
well-bonded durable composite.
As previously noted, the method of the invention is not limited in its
operability to the use of any particular composition of brittle layer
(ceramic) material. Instead, any of a relatively broad range of ceramic
and glass-ceramic materials may usefully be selected for incorporation in
these microlaminated systems. Further, the interlayered or second layer
materials need not be limited in their composition only to metals such as
steel or nickel, but could instead comprise any of a variety of other
metal, metalloid, intermetallic or ceramic materials. The latter may
comprise glass-ceramic, polycrystalline ceramic, or even glass
interlayers, if desired.
Table 4 below sets forth examples of microlaminated composites fabricated
of some of these alternative materials in accordance with the invention.
Included in Table 4 for each of the composites provided are an
identification of the brittle and second layer materials used, the numbers
and thicknesses of the layers, and the consolidation conditions of heat
and pressure used for composite consolidation. All composites were square
composites in the 19-25 mm size range.
TABLE 4
______________________________________
Ex- Consolid.
ample Brittle Layer
Interlayer Consolid.
Temp.
No. (No.-Thick.) (No.-Thick.)
Pressure
(.degree.C.)
______________________________________
20 alumina + borosilicate
19.3 kPa
685.degree.
15% ZrO.sub.2
glass 7-90 .mu.m
(2.8 psi)
8-40 .mu.m
21 zirconia(TZ-2Y)
borosilicate
22.9 kPa
685.degree.
7-15 .mu.m glass 6-90 .mu.m
(3.33 psi)
22 cordierite g/c.
silicon 24.1 kPa
850.degree.
4-40 .mu.m (1-xtal.) (3.5 psi)
5 @ 290 .mu.m
23 cordierite g/c.
copper 24.1 kPa
850.degree.
6 @ 40 .mu.m 7 @ 127 .mu.m
(3.5 psi)
25 CAS g/c. nickel 16.5 kPa
1300.degree.
7 @ 44 .mu.m 8 @ 51 .mu.m
(2.4 psi)
26 ZrO.sub.2 ( + Al.sub.2 O.sub.3 +
molybdenum 16.5 kPa
1450.degree.
CAS), 7 @ 50 .mu.m
8 @ 25 .mu.m
(2.4 psi)
27 CAS + Al.sub.2 O.sub.3
410 stainless
16.5 kPa
1300.degree.
7 @ 50 .mu.m
8 @ 51 .mu.m
(2.4 psi)
28 CAS + ZrO.sub.2
nickel 16.5 kPa
1300.degree.
7 @ 50 .mu.m 8 @ 51 .mu.m
(2.4 psi)
29 zirconia(TZ-2Y)
copper 16.5 kPa
1000.degree.
11 @ 50 .mu.m
12 @ 25 .mu.m
(2.4 psi)
30 alumina/MgO aluminum 17.2 kPa
550.degree.
8 @ 36 .mu.m 7 @ 100 .mu.m
(2.5 psi)
31 alumina/MgO tungsten 25.4 kPa
1600.degree.
7 @ 50 .mu.m 8 @ 100 .mu.m
(3.7 psi)
32 alumina + niobium 25.4 kPa
1600.degree.
15% ZrO.sub.2
6 @ 250 .mu.m
(3.7 psi)
5 @ 30 .mu.m
33 alumina/MgO tantalum 25.4 kPa
1600.degree.
7 @ 50 .mu.m 8 @ 12.5 .mu.m
(3.7 psi)
34 ZrO2 + zirconia 11.7 kPa
1500.degree.
20% Al203 (TZ-2Y) (1.7 psi)
24 @ 10-15 .mu.m
23 @ 10-15 .mu.m
______________________________________
Inspection of the microlaminated composites reported in Table 4 indicated
that well-bonded composites were achievable in all of the systems shown.
In the glass interlayer composites (Examples 20 to 21), however, which
utilized Corning Code 0211 alkali zinc borosilicate glass interlayers,
plastic deformation of the flexible ceramic layers was not achieved at the
consolidation temperatures used, so the bonding realized was not
high-temperature deformation bonding but instead conventional
glass-to-ceramic sealing.
In Examples 22 and 23, the flexible ceramic (brittle) layers were
cordierite glass-ceramic (g/c.) sheets produced by the crystallization of
powdered magnesium aluminosilicate glasses. These were found to produce
well-bonded composites with both single-crystal silicon (Example 22) and
copper metal (Example 23), thus showing promise for integrated circuit and
electronic substrate applications.
Examples 25-28 utilized sintered ceramic foils based on calcium
aluminosilicate (CAS) glass-ceramics (g/c.) wherein anorthite constituted
the principal crystalline phase. In the nickel-containing microlaminate
(Example 25) bonding was excellent even though there was some evidence of
reduction of the ceramic at the ceramic/metal interfaces.
In Example 26, the CAS glass-ceramic (17 wt. %) was present as an additive
along with alumina (17 wt. %) in a yttria-stabilized zirconia base, this
combination ceramic showing excellent bonding to the molybdenum foil.
Examples 27 and 28 utilized ceramic foil containing 60% and 50% CAS
glass-ceramic, respectively, with the remainder of the foil composition
comprising the oxides shown. These samples also showed excellent
interlaminar adherence to the stainless steel and nickel substrate layers.
Example 29 showed excellent bonding between yttria-stabilized zirconia and
copper, while Example 30 combining alumina (with 1% of MgO sintering aide)
and aluminum metal also showed good bonding. However, in the latter case,
the alumina ceramic could not plastically deform at the bonding
temperature used. Thus flatness in the ceramic foil is particularly
important in microlaminates such as these, in order to minimize unbonded
areas and the need for extensive plastic reshaping of the deforming (less
refractory) layer.
Example 31 showed that excellent bonding is obtainable in a
tungsten/alumina (semi-brittle/brittle) composite structure. The composite
of that example was sufficiently durable to be edge-ground to a 90.degree.
cutting edge, such as would be utilized for an edge on a cutting tool.
FIG. 2 of the drawing is an electron photomicrograph of an edge-ground
composite of this composition, the bar representing a dimension of 200
.mu.m, wherein the 90.degree. edge can be observed along the top boundary
of the layered structure. The complete absence of delamination of the
tungsten sheets (dark layers) from the alumina foil (light interlayers) is
readily apparent.
Example 32, comprising a laminate of alumina/zirconia (alumina with 15%
TZ-2Y) with niobium also exhibited generally good bonding. Example 33
comprising tantalum foil, which demonstrated good bonding in an initial
evaluation, did exhibit some edge delamination when subjected to abrasive
grinding and polishing.
A typical example of a ceramic-ceramic (brittle/brittle) microlaminate is
provided by Example 34 of the Table. That laminate, a bar approximately
5.7 cm in length formed of interlayered zirconia foil (TZ-2Y) and
zirconia/alumina foil (20% Al2O3; 80% TZ-3Y zirconia), exhibited excellent
bonding after lamination at 1500 .degree. C. for 2 hours in air.
FIG. 3 of the drawing is an electron photomicrograph of a cross-section of
a microlaminate formed in accordance with Example 34, taken under dark
field illumination and wherein the bar represents a dimension of 20 .mu.m.
The layer structure of this composite is well-formed, with no microcracks
observed perpendicular to or through the layers of the composite despite
the brittle nature of the laminae and the relatively high thermal
expansion mismatch therebetween. The few voids or separation cracks
observed at the layer interfaces were attributed to cracking during edge
grinding and/or a slightly less than optimum bonding temperature,
Another particular advantage associated with metal/ceramic microlaminated
composites provided in accordance with the invention relates to the
versatile joining characteristics thereof, particularly in the difficult
area of joining ceramics to metals. The following example is illustrative.
EXAMPLE 35
A twenty-five layer microlaminate comprising twelve sheets of zirconia (2
mole % yttria) alternatingly interleaved with thirteen sheets 410 series
stainless steel foil was fabricated by consolidating the sheet stack at a
temperature of 1400 .degree. C. and a pressure of 16.4 kPa (2.38 psi.).
The peak consolidation temperature was maintained for only about a minute
before the cooling cycle was initiated.
The resulting composite, exhibiting good interlayer bonding, was set
endwise against a small steel plate to form a "T" junction, and was silver
soldered to the plate. A second small steel plate was then silver brazed
to the opposite end of the microlaminate sample to form a small "I" beam
sample. Repeated vigorous flexing and bending did not result in
delamination of the composite or separation of the composite from either
of the steel endplates.
The significance of the above result is that, in addition to having utility
alone as a durable composite material, these composites can also serve as
joining structures to form complex assemblies of components otherwise
incompatible with each other and with more conventional bonding media. Of
course, for these and other special purpose applications, microlaminated
composites which are not of homogeneous composition, but instead are of
asymmetric composition to achieve gradations in physical and/or chemical
properties from one end or surface of the composite to the other, may well
be preferred.
The few instances of poor interlayer bonding observed in the various
microlaminated composites evaluated were generally attributable to
chemical incompatibility between the ceramic foils and substrate layer
materials selected. Thus an alumina/yttrium microlaminated sample showed
regions of interlayer reaction at the metal/ceramic interfaces, these
reactions interfering with metal-ceramic adherence. In the case of a
magnesium/alumina composite, magnesium oxidation during consolidation from
oxygen in the furnace environment prevented good interlayer bonding from
being achieved.
As previously noted, it has been found particularly important in the
practice of the invention that the ceramic foils used to provide the sheet
stacks for the composites be sufficiently sintered to provide a unitary,
flexible and strong ceramic film prior to use. Our attempts to provide
ceramic/metal composites from sheet stacks wherein, instead of sintered
ceramic foils, green (unfired) ceramic powder tapes were laminated with
metal foil layers to provide the sheet stacks, provided products which
were not integral and well-bonded composites.
In these trials it appeared that the sintering of the green tapes during
the hot pressing of the sheet stacks interfered extensively with bonding
to the foil, producing only partially bonded assemblies. We attribute
these results to the relatively high degree of shrinkage which occurs in
the course of in-plane densification of green ceramic tapes. This
shrinkage appeared to cause tape ripping, or in some cases rumpling of the
metallic interlayers in the composites, making the achievement of high
temperature deformation bonding essentially impossible.
Applications for the microlaminated composites of the invention are
numerous, ranging for example from applications such as consumer knives
and industrial cutting tools to high temperature air-frame structures,
turbine and other heat engine parts including corrosion and wear resistant
coverings, and many other products. Thus the foregoing examples and
description are intended to be merely illustrative of the many
combinations and variations of procedure and composition which may be
resorted to by those skilled in the art within the scope of the appended
claims.
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